Display devices comprising liquid crystalline compounds have widely been employed for the displays of watches, tabletop calculators, word processors, TV sets, or the likes. These display devices employ the optical anisotropy and dielectric anisotropy of liquid crystalline compounds.
Liquid crystal phase exists between a solid phase and a liquid phase, and the liquid crystal phase includes a nematic phase, smectic phase, and cholesteric phase as its specific form. Among which, the nematic phase is most in use at present for display devices. As the specific type of display mode, TN (twisted nematic) mode, DS (dynamic scattering) mode, GH (guest-host) mode, DAP (deformation of aligned phases) mode all of which exhibit electro-optic effects have been developed. Meanwhile, display of liquid crystals in color is further progressing lately, and particularly devices of TFT (thin-film-transistor) mode and STN (super twisted nematic) mode both of which form a class of TN mode have now become a main current.
Whereas many liquid crystalline compounds including ones which are currently at a stage of research are known up to now, compounds which can be filled in display devices as a single compound to actually use as liquid crystal composition have not been developed. This is because whereas liquid crystalline compounds expected to be used as a material for display devices must exhibit a liquid crystal phase at room temperature, at which display devices are most often used, as well as in a temperature range as wide as possible with the room temperature being its center; must be sufficiently stable against environmental factors under conditions in which the devices are used; and must have physical properties sufficient to drive display devices, no compounds which can satisfy these requirements by a single compound have been found. Accordingly, it is an actual circumstance at present that several kind or several tens kind of liquid crystalline compounds, and non liquid-crystalline compounds when necessary, are mixed to prepare liquid crystal compositions having the required characteristics. It is considered to be necessary that these liquid crystal compositions are stable against moisture, light, heat, and air which are usually present under the conditions in which display devices are used; are stable also against electric field and electromagnetic radiation; and are chemically stable against liquid crystalline compounds to be mixed. Also, it is considered to be necessary that liquid crystal compositions have an appropriate value of physical properties such as optical anisotropy (.DELTA.n), dielectric anisotropy (.DELTA..epsilon.), viscosity (.eta.), electroconductivity, and ratio of elastic constants (K.sub.33 /K.sub.11 ; K.sub.33 : bend elastic constant, K.sub.11 : splay elastic constant) depending on the display modes and the shape of display devices. Further, it is important that each component in liquid crystal compositions have a good mutual solubility to one another.
It is considered to be necessary that the liquid crystal compounds used in displays of STN mode particularly have a low viscosity, large K.sub.33 /K.sub.11, and wide temperature range of a liquid crystal phase in addition to the properties described above. Further, a low threshold voltage (Vth) is quite recently considered to be necessary accompanied with driving of display devices at a low voltage.
That is, low viscosity is an essential property for achieving a high speed response. Also, large K.sub.33 /K.sub.11 makes the change of transmittance around V.sub.th steep and makes the production of display devices of a high contrast possible. As the compounds exhibiting a large K.sub.33 /K.sub.11, ones having an alkenyl group as a lateral chain, for example, compounds expressed by the formula (a) disclosed in Japanese patent publication No. Hei 4-30382 and compounds expressed by the formula (b) disclosed in Japanese patent publication No. Hei 7-2653 are already known. However, since these compounds have a strong tendency to develop a smectic phase by themselves, they sometimes separate crystals or develop a smectic phase particularly at low temperatures when used as a component of liquid crystal compositions. Thus, the compounds can not be said to be good in mutual solubility at low temperatures.
Besides, whereas these compounds exhibit some extent of viscosity reducing effect on liquid crystal compositions to which the compounds are mixed, the compounds have a defect of lowering clearing point, and have a problem that they remarkably lower .DELTA..epsilon. when added to liquid crystal compositions having a high .DELTA..epsilon. used for the purpose of driving the devices at low voltages, since the compounds themselves have a remarkably low .DELTA..epsilon..
As compounds having a structure similar to that of the compounds of the present invention, ones expressed by the formula (c) are known as shown in U.S. Pat. No. 5,183,587. However, these compounds have a problem that heat stability is very poor. ##STR2##
In addition, compounds having only an alkenyl group or a halogen substituted alkenyl group as a terminal group in the molecule are disclosed, for example, in Laid-open Japanese Patent Publication Nos. Sho 61-83136 and Sho 63-142091. However, these compounds are insufficient for collectively achieving the effect of maintaining or raising clearing point and the effect of not lowering .DELTA..epsilon. (not raising V.sub.th) of liquid crystal compositions. Besides, these compounds have a defect in solubility to other known liquid crystal compounds, particularly in that at low temperatures.
From such circumstances in conventional technology, development of liquid crystalline compounds having a low viscosity, large K.sub.33 /K.sub.11, excellent heat stability, wide temperature range of a liquid crystal phase, comparatively low V.sub.th, and improved solubility at low temperatures has been desired.